1. Field of the Invention
The present invention relates to an inflatable cuff for oscillometric blood pressure measurement, which is adapted to be worn on a body portion of a living subject, and particularly to a cuff having a first inflatable bag for pressing the body portion and a second inflatable bag for sensing a pulse wave.
2. Description of Related Art
In general, an oscillometric blood pressure measuring device determines a blood pressure based on pulse waves continuously received by the cuff wound around a body portion during a slow deflation to reduce a pressure by the cuff.
The device adopts a static pressure of the inflatable bag as a systolic pressure, at a rising point of an envelope of amplitudes of the pulse waves, that is, a point where the amplitude of the pulse waves suddenly rises.
The device may determine an indefinite systolic pressure with an indefinite rising point of the envelope of the amplitudes of the pulse waves when the cuff has only one inflatable bag used both for pressing the body portion and for sensing the pulse wave. This is attributed to a propagation of pulsations, which are generated from an arterial vessel around an upstream end of the cuff to the cuff by an influence of the pulsations gradually increasing with the decreasing pressure of the cuff approaching the systolic pressure even when the pressure of the cuff is over the systolic pressure. The device tends to provide with the indefinite rising point of the envelope of the amplitudes of the pulse waves with the cuff wound, especially, around a portion where it is difficult to fully stop a blood flow in the arterial vessel by the cuff, such as an ankle.
To solve the above problem, JP 05-269089 A, U.S. Pat. Nos. 6,346,083 and 6,497,668 disclose a cuff having an inflatable bag for pressing a body portion so as to stop a flow of blood in an arterial vessel and an inflatable bag for sensing a pulse wave from the arterial vessel. The cuff disclosed in these references includes an outer bag for pressing the body portion and an inner bag for sensing the pulse wave. A device using this cuff provides with a relatively definite systolic pressure with a relatively definite rising point of the envelope of the amplitudes of the pulse waves sensed by the inner bag, because the inner bag is substantially located in the center of and on a body portion side of the outer bag and no pulsation directly propagates to the inner bag when the arterial vessel pulsates again around an upstream end of the outer bag.
To an outer side of the inner bag of the references, a shield plate is adhered so as to prevent oscillation of the outer bag from propagating to the inner bag. The shield plate causes the device to provide with a more definite systolic pressure preventing noises in the outer bag from propagating to the inner bag.
It is a disadvantage of the inventions disclosed in the references that noises produced by a relative slide motion of the shield plate and the bag for sensing the pulse waves, that is, by friction between the shield plate and the bag for sensing, cause inaccuracy in blood pressure measurement by propagating with signals output from the bag for sensing during slow deflation of the bag for pressing and the bag for sensing for blood pressure measurement because the shield plate is disposed outer side of the inner bag, that is, the bag for the pulse wave in the above references.